As known in the art, an integrated circuit (IC) die is attached to a workpiece such as a package substrate, lead frame or a carrier during packaging of IC devices. The package substrate or carrier is generally made of an electrically insulating (i.e., dielectric) material, for example, ceramic or plastic. The process by which the IC die is attached to the workpiece is known as die bonding. There are different techniques of die bonding including, for example, eutectic bonding, preform bonding, and adhesive bonding.
Adhesive bonding includes using an adhesive paste, also known as a die attach or die bonding adhesive, to attach the IC die to a workpiece. The bonding adhesive can be dispensed on the workpiece and the IC die placed over it. The adhesive may include an epoxy monomer or another suitable liquid carrier, and generally includes a suspension of filler particles such as metal filler particles in the carrier. As known in the chemical arts, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large (generally at least 1 μm in size) for sedimentation. The internal phase (solid) is dispersed throughout the external phase (liquid) through mechanical agitation for mixing before loading into the die bonder. Unlike colloids, suspensions have the tendency to settle over a period of time which results in separation of the solid particles and the liquid.
The metal particles can be silver particles, although other metals or electrically conductive materials may be used, e.g., gold, nickel, copper, or graphite, etc. It is generally desirable to load the metal particle content as high as possible in order to obtain the highest possible thermal and electrical conductivity. The particles are generally denser as compared to the liquid carrier. Following die attach, a heated curing step is generally used that polymerizes the liquid carrier. Curing the carrier provides adhesion to make a bond to both the IC and the workpiece with the required mechanical strength, and the metal particles provide electrical or/and thermal conductivity. The adhesive can thus be electrically conductive or non-conductive, and thermally conductive or non-conductive.
In a known technique of adhesive bonding for an adhesive comprising metal filler particles in a carrier, the uncured adhesive is mixed well while in the dispenser which includes a reservoir for the bonding adhesive by mechanical agitation provided by a stand-alone mixer, such as a combined centrifuge mixer, to resuspend the filler to provide a homogeneous, uniform suspension. After mixing, the dispenser is then secured to the die bonder apparatus. While in the die bonder apparatus the suspension is then injected from the dispenser onto the workpiece, which is followed by attachment of the IC die. The dispenser may include a syringe having an end aperture.
During normal production, the metal or other particles tend to remain in suspension because dispensing includes some X-Y motion which helps prevent precipitation so that separation of the denser filler particles that can otherwise cause delamination of the die attach adhesive from the IC device is typically not a significant problem. As a method of process control, X-ray inspection can be used after die bonding to check for delamination.